Field of the Invention
This invention relates to polymeric particles of narrow sized distribution and to a method of preparing the same. More particularly, the invention relates to polymeric particles suitable for use as electrostatographic toners, matte beads for forming matte surfaces on photographic elements and as polymeric pigment particles.
There are many applications for powdered polymeric particles where it is important that the particles have a narrow size distribution. One such application is where the particles are used as electrostatographic toners. In such toners the particles can function, for example, as the sole toner component to form toner images or as binders for other toner addenda such as colorants and charge control agents. The electrostatographic toners are in the form of particles that are subject to electrostatic forces and other forces that affect the particles differently depending on their size, and, in order to obtain good copies, it is necessary that all of the particles be affected in substantially the same way in the copying process. This means that the particles must have a narrow size distribution. While there are many processes that produce polymer particles, few produce such particles having a narrow size distribution. If the particles do not have a narrow size distribution, it may be necessary to size them by passing the particles through sieves. This is an expensive process which significantly adds to the cost of the electrostatographic toner.
A well-known process that provides polymer particles having a narrow size distribution employs a solid colloidal stabilizer to control both particle size and particle size distribution. One example of this type of process is described in U.S. Pat. Nos. 2,932,629 and 4,148,741, which pertain to a suspension polymerization process where a solid colloidal stabilizer such as silica is used to limit the coalescence of droplets containing polymerizable monomer in an aqueous medium. In that process, a water-immiscible polymerizable liquid is sheared to form small droplets suspended in aqueous medium containing a water dispersible water-insoluble solid colloid such as silica as the suspension stabilizer. The concentration and size of the colloid determines the size of the droplets. The colloid performs this function by adhering to the droplets at the water/monomer interface to form a layer on the surface of the droplets. After monomer droplets have coalesced with other droplets and have grown to a particular diameter, the presence of the layer of colloidal stabilizer particles on the surface of the droplets prevents them from further coalescing and increasing in diameter. In this way, all of the droplets tend to grow to approximately the same diameter, so that upon polymerization the resulting polymer particles have a narrow size distribution.
A second example of a process that provides polymer particles having a narrow size distribution using a solid colloidal stabilizer comprises forming a solution of polymer in a solvent that is immiscible with water, dispersing the polymer/solvent solution in an aqueous medium containing silica as the solid colloidal stabilizer, removing the solvent, dehydrating and drying the resulting particles. For ease in distinguishing this type of process from the aforementioned "suspension polymerization" process, it is referred to hereinafter as the "polymer suspension" process. This type of process is described in U.S. Pat. No. 4,833,060, issued May 23, 1989 assigned to the same assignee as this application and herein incorporated by reference.
The use of solid colloidal stabilizers such as silica to control particle size and size distribution of the resulting polymers has some disadvantages. For example, such solid colloidal particles can impart surface characteristics to the polymers that are incompatible with the intended use. Thus, if silica is used as the colloidal stabilizer in the preparation of polymer particles for use as electrostatographic toners it must be removed from the particles because silica adversely affects the triboelectric properties and the fixing characteristics of the toner. The removal of silica from the polymer particles requires several additional processing steps that significantly add to the cost of the toner. Furthermore, stabilizers such as silica are difficult to remove as is evident from U.S. Pat. No. 4,835,084, assigned to the same assignee as this application and incorporated herein by reference.
Another technique for preparing polymeric particles is that described in U.S. Pat. No. 4,965,131 issued Oct. 23, 1990 and assigned to the same assignee as the immediate application. This technique results in polymeric particles surrounded by smaller polymeric particles. These smaller particles serve as a colloidal suspension agent in processes similar to that described above. This process has the advantage in that the surface characteristics of toner particles, for example, can be designed into the particles by choice of the polymeric suspension agents. However, it is not the intent of this technique that toner particles be prepared by providing a colloidal suspension agent that is readily removed from the core particle.